The present invention, is directed generally to amplifiers, and more specifically to an output buffer which is electrically alterable to be converted between active pull-up and open drain nonvolatile configurations.
The prior art has included convertible amplifiers which generally have mechanical switches such as manually operated switches or relays or transistor switches to select a desired configuration. These circuits have not been used in integrated circuits.
Integrated circuits having field effect transistor output buffers are commonly configured as open drain or active pull-up. The standard approach taken by semiconductor manufacturers is to include within the circuitry of the buffers, the components necessary to provide the active pull-up configuration and by means of a metal option create the open-drain configuration. Two metal masks are required to provide the simple option. Also, two sets of test programs are required at the final test area for all D.C. tests. All tests must separate open-drain from active pull-up configurations due to inevitable product mix at both the wafer, assembly and final test rescreen levels. This leads to serious logistics problems of maintaining product identity in the wafer fabrication process as well as in the final test areas. Thus there exists a need for a methodology of configuring amplifiers or output buffers in a single configuration which may be electrically altered after final testing to allow selection of a desired permanent configuration. The same problem exists for bipolar transistor output buffers with the necessity of providing push-pull or open collector configuration.